1. Field of the Invention
This invention relates generally to a seismic systems, and more particularly to seismic systems used in the hydrocarbon exploration and mining industries.
2. Description of the Prior Art
Terrestrial seismic data acquisition systems are well known in the art. An array of geophones are positioned across a geographical region, typically in a grid pattern, for measuring seismic vibrations. The precise location of each geophone must be known and is typically ascertained by a separate positioning survey. A seismic vibration source is activated, and the geophone measurements are recorded, sometimes over a period of several hours to several days or weeks, collected, and subsequently processed to determine the structure of the earth at that geographical region.
In many seismic systems, multi-channel digital recorders are connected to a number of input geophone channels for sampling and converting the analog geophone outputs to digital format, which are recorded. It is often preferred that the analog-to-digital conversion occurs in the field close to the geophones to minimize degradation of the low-level geophone signals.
It is necessary to ensure that all of the numerous digitized seismic channels can be precisely correlated to a common time standard for analysis. Sampling time errors result in wrong sets of data being analyzed for each instant of time and are perceived as noise. The noise created by sample timing mismatch is both time and frequency dependent, because the errors created depend on the slew rate of the signal at each sample instant. Large timing errors cause binning problems in depth point processing and result in significant depth errors.
In many prior art seismic recording systems, the seismic channel data are analyzed strictly according to their sequential sampling order, and timing errors are minimized by ensuring simultaneous triggering of all recorders to begin sampling. Triggering the digital recorders may be performed by hard-wired or wireless radio frequency (RF) control.
In hard-wired systems, control and power are provided by cabling that connects the digital recorders to a control interface. This type of system has an additional advantage of having a conductive signal path to transmit seismic data from the numerous digital recorders for central collection, typically at a control vehicle. However, the cabling is heavy, and weight is a significant cost in deploying a seismic system. Also, the cables are subject to damage by being crushed under passing vehicles or being chewed by rodents and livestock, for example.
In wireless systems, the weight of the cabling between digital recorders is eliminated, but the weight savings is offset by batteries used to power the digital recorders and RF receivers used for triggering wireless systems. Wireless systems also typically employ RF transmitters in the digital recorders for transmitting seismic data to a central location for collection and processing. Large wireless seismic survey systems require a significant amount of RF bandwidth and a fairly large antenna at the control vehicle. Radio wave propagation paths may be obstructed by terrain, vegetation or structures. Furthermore, battery life limits the time a system may remain in the field, even in standby conditions, and creates logistical difficulties in deploying large seismic system arrays.
FIG. 1 illustrates a “wireless” seismic recording system of prior art in which strings of geophones (1) are connected to multi-channel digitizer modules (2), for example, six geophones per string and four channels per recorder. Because of the low-level signals produced by analog sensors, the digitizer modules (2) are each located in close proximity to their attached geophones. The digitizer modules (2) include preamplifiers and analog-to-digital converters to digitize the geophone signals into digital data. The seismic digital data are then in turn transmitted by the digitizer modules to a data acquisition module (3) via electrical cabling (4). Depending on the telemetry techniques employed, the data acquisition modules (3) may be located a significant distance away from the digitizer modules (2).
In a given survey, a number of data acquisition modules (3) are used, each having its own corresponding digitizer modules (2) and geophone strings (1) to form an independent cell within the wireless system. Two such cells are represented in FIG. 1. Each data acquisition module contains memory for recording the digital seismic data from its corresponding family of digitizer modules, and the data acquisition module supplies power, control, and synchronized clock signals to the digitizer modules. Each data acquisition module has a battery, and it may also include a radio frequency transmitter for broadcasting collected seismic data to a control vehicle, for example. Timing synchronization is maintained between the geophone channels by including a global positioning receiver within each data acquisition module (3). Each data acquisition module (3) continuously processes satellite navigation data to provide a common time standard among the units. Other prior art seismic systems, for example, U.S. Pat. No. 5,978,313 issued to Longaker, U.S. Pat. No. 7,269,095 issued to Chamberlain et al., and U.S. Patent Publication No. 2008/0021658 in the name of Pavel et al., disclose seismic systems that employ satellite navigation system receivers for establishing synchronization among the recording units, but no provision is made for idling the satellite receiver to conserve power.
As the price of oil has increased and the cost of computer processing power has decreased, it is desirable to increase the number of geophones or other sensors used in a survey to cover larger areas and provide higher resolution surveys. For a three dimensional survey, many thousands of geophones or other sensors may be deployed over many square miles. Synchronization, power management, and deployment of digital recorders on this scale can be problematic. Although the data acquisition modules of the system of FIG. 1 are wireless, a large amount of cabling (4), with its inherent disadvantages, is still required between the data acquisition modules (3) and the digitizer modules (2). A wireless system of energy-efficient, high-capacity digital recorders collocated near the geophones with the digitizing circuitry would advantageously reduce the amount of cabling required.
3. Identification of the Objects of the Invention
A primary object of the invention is to provide a seismic data acquisition system including field-deployable digital recorders for capturing seismic data that have integral satellite receivers for synchronizing the timing between recorded seismic data from multiple recorders, thus eliminating the cost of deploying traditional timing/triggering systems.
Another object of the invention is to provide a wireless seismic acquisition system wherein the digital recorders are collocated with digitizing electronics near the analog sensors, thus minimizing the volume, weight and cost of cable to deploy and maintain.
Another object of the invention is to provide a wireless digital seismic recorder with an independent acquisition clock circuit that is used to sample and to time stamp recorded seismic data and a satellite receiver that is activated only periodically to adjust the acquisition clock circuit. By not continuously processing satellite navigation signals, power requirements are reduced.
Another object of the invention is to provide an intelligent digital seismic recorder that periodically adjusts a local acquisition clock circuit to a satellite time standard and that measures the acquisition clock drift and temperature changes and adjusts clock cycle times to maximize periods between synchronization while maintaining the acquisition clock circuit within a given error tolerance.
Another object of the invention is to provide a seismic data acquisition system having field-deployable digital recorders that periodically record time stamps with seismic data and that are equipped with large non-volatile memories to eliminate the need for centralized triggering.
Another object of the invention is to provide a seismic data acquisition system arranged for transferring recorded seismic data to collection computers in the field during surveys, after the data is collected.
Another object of the invention is to provide a seismic data acquisition system having field-deployable digital recorders equipped with low-power-consumption electronics and that are designed and programmed to automatically and selectively de-energize or idle system components when not required for use, thus extending the usable time in field and reducing operating costs.
Another object of the invention is to provide a seismic data acquisition system having field-deployable digital recorders which employ dynamic voltage control techniques to lower computer processor power consumption by idling the processor during periods of lower computational demands.